Latitude/Longitude
The Latitude/Longitude system is the most commonly known coordinate system. It breaks the earth into 360 degrees along the equator (180° East and 180° West) which is the Longitude component. It also breaks the earth into 180 degrees in the North/South direction (90° North and 90° South), which is the Latitude component. Each degree can be divided into 60 minutes and each minute can be further sub-divided into 60 seconds.
Specifying a Latitude/Longitude location using the Degrees, Minutes and Seconds gives the Degree Minutes Seconds (DDD MM SS.SSSS) format. Alternatively, a location may be expressed using Degrees and Minutes, where the Minutes include a decimal portion. This format is called Degree Minutes (DDD MM.MMMMM). In computer applications, it is often easier to work with a coordinate as a single value, so locations are often expressed using just the Degrees, with the minutes and seconds expressed as a decimal. This is known as Decimal Degrees. Typically, when using Degree Minutes and Degree Minute Seconds notation, North/South and East/West are displayed explicitly, while in Decimal Degrees, positive values indicate North and East, while negative values indicate South and West (see examples).
The following are examples of ways the same location can be displayed in Longitude/Latitude:
- Decimal Degrees: 49.25438 -123.19807
- Degree Minutes: 49° 15.263 N 123° 11.962 W
- Degree Minutes Seconds: 49° 15' 15.7788" N 123° 11' 57.7212" W
Universal Transverse Mercator
The Latitude/Longitude system is good for looking at locations on a global scale but is often not as easy to work with when working in a small area. For example, it is not easy to determine the distance between two points given in Latitude/Longitude format. Another coordinate system, used for small area mapping is the UTM (Universal Transverse Mercator) system. In UTM, the earth is divided into 60 zones along the equator, and 22 bands in the North/South direction. Each zone basically has its own coordinate system based on the centre (central meridian), of the zone. The central meridian is given a value of 500,000 meters east. The East/West coordinate, or Easting in UTM, is then given in relation to the central meridian. For example, if a point is 250,000 meters east of the Central Meridian, its Easting value would be 500,000 + 250,000 or 750,000. If a point is 100,000 meters West of the central meridian, its Easting value would be 400,000.
The North/South coordinate in UTM is known as the Northing. In the Northern Hemisphere, the Northing is the number of meters north of the equator. In the Southern Hemisphere, to avoid negative numbers, the equator is given a value of 10,000,000 m, and Northings are given in relation to that. For example, a point 1,500,000 meters South of the Equator would have a Northing of 8,500,000. Since UTM expresses locations in meters it is easier to determine differences in distance. For example, if one point has an Easting of 485,492, and another point has an Easting of 486,501, you can tell that the second point is approximately 1,000 m or 1 km further east. To display a location in UTM, then, the Zone, Easting and Northing values need to be displayed. The latitude band is not mandatory, but should be displayed for clarity; if the band is not displayed, then the Northern or Southern hemisphere must be indicated.
The above Latitude/Longitude, converted to UTM would be displayed as:
- 10U E 485587 N 5455754
which means is in Zone 10, Band U, has an Easting of 485492 and a Northing of 5455754.
Because UTM tries to take a spherical earth and converts it into a rectangular coordinate system, a mathematical model of the earth needs to be used. Several models or datums exist and different ones work better for different parts of the earth. The most up-to-date and commonly used worldwide Datum is WGS84 (World Geodetic System). When in doubt this is the best one to use, but if using local maps, it is critical to choose the datum that matches the datum the map was made with.
